From the standpoint of resource and energy savings, for example in the automotive industry, it is desirable to reduce vehicle body weight. However, attempts to reduce weight by reducing the thickness of metal panels result in deterioration in strength. For example, in the case of quarter panels for automobiles, such attempts have created problems such as low strength at various points (particularly flexural strength). Insufficiently rigid outer panels buckle and bend with minimum applied stress. In addition, thin outer panels exhibit poor impact absorbing properties.
In order to solve such problems, it is necessary to develop a suitable way to reinforce such structures. Reinforcement with a heavy metal panel is contradictory to the purpose of vehicle weight reduction. Hence, it has been proposed to reinforce the outer panels entirely or partly with lightweight resin materials.
Reinforcement comprised of thermosetting adhesive layers and a reinforcing material has been applied to steel panels for use in the production of the body of automobiles. Such reinforcement provides increased strength against force applied to the outside surfaces of the automobile body.
For example, U.S. Pat. No. 4,369,608 describes a door for an automobile wherein a main reinforcing member made of a thermosetting resin is bonded to the inner side of an outer metal panel of the door to improve the strength and rigidity of the door. A multilayer sheet-like auxiliary reinforcing member, preferably made of a glass fabric, is bonded to the main reinforcing member.
Typically, the reinforcing sheet is adhered under pressure, for example to the back surface of the metal panel to be reinforced, and thereafter cured by usual heating methods, e.g. by heating in a hot air circulation type heating oven, in an infrared ray heating oven or with a radio frequency induction heating coil. This heat curing treatment can be carried out simultaneously at a stage in a vehicle assembly line at which painted metal panels, having a reinforcing sheet thereon, are baked to cure the paint.
The reinforcing sheet may be premolded to conform its shape with that of the metal panel to be reinforced. Preferably, the reinforcing sheet has sufficient flexibility so as to conform to the shape of the metal panel upon its application to the metal panel without premolding.
However, the use of conventional reinforcing adhesive sheets containing thermosetting resins for reinforcing a metal panel has shortcomings.
Conventional reinforcing adhesive sheets must be reasonably thick in order to obtain the desired strength and impact resistance. Simply bonding thin resin sheet material to the metal panel may not provide adequate flexural reinforcement and impact resistance. If the thickness of the resin sheet is increased, the weight will also be increased. If packing is used between the resin sheet material and the panel to insure sufficient thickness, the reinforcing member will conform poorly to the shape of the panel, or will have a complicated structure. Wave-like or bead-like projections provided on the main reinforcing member, which projections function as ribs of the metal panel to be reinforced, cause similar disadvantages and only moderate increases in flexural reinforcement.
It can thus be readily appreciated that provision of a reinforcing sheet which provides improved flexural reinforcement and impact resistance to a cold rolled steel panel as compared to conventional reinforcing sheets and eliminates the previously discussed problems would be a highly desirable advance over the current state of panel reinforcement technology.